The word "ceramics" is derived from the Greek word keramos ("burnt stuff"), and traditionally encompasses most inorganic materials except metals. Traditional ceramics are derived from the base material clay, an essentially hydrated compound of aluminum and silicon H.sub.2 Si.sub.2 O9, containing more or less foreign matter such as ferric oxide, silica (as sand) and calcium carbonate (as limestone). An important and growing area of the ceramic field, is ceramic composites. This area covers a variety of combinations such as sapphire, aluminum oxide, whiskers in metals, metal-bonded carbides used in the machine tool industry, and directionally solidified two-phase ceramic systems (wherein an oriented fibrous second phase is grown in a primary matrix phase to maximize in a selected direction various important characteristics, such as minimum long-term, high-temperature creep).
Ceramic composites have a variety of uses such as in electrically insulating integrated circuit packages and substrates, where aluminum oxide dominates the field. The lightweight characteristics of ceramic composites are being touted as valuable for high-temperature engines for future automobiles. Further, the hardness of certain ceramic materials may permit their usage as cutting tools for abrasive uses such as grinding.
A chemical technique for producing ceramic composites that is used in the present invention is known as sol-gel processing. Unlike traditional glass and ceramic processing techniques in which powders are reacted or melted at high temperatures, the sol-gel process relies upon hydrolysis and condensation reactions in liquid solution at near ambient temperatures. Typically, a precursor, solvent, water and a catalyst are mixed together in solution. The sol is a suspension of small particles (colloids) ranging in size from about 1 to 100 nanometers. On partial loss of the liquid phase, the sol increases in viscosity, eventually becoming a wet, porous gel. The gel is composed of an interconnected porous matrix phase which initially contains alcohol, unreacted precursors and water. This wet, porous gel is subjected to additional processing stages where it is dried and densified to produced desired ceramic composite characteristics. For example, the wet, porous gel can be simply dried to form a porous solid or further converted into a dense glass or ceramic.
Aluminum oxide is a material commonly used in electronic substrates, i.e., hybrid circuit boards for integrated circuits. Aluminum oxide is an excellent insulator having a resistivity which is greater than 10.sup.14 Ohm-cm. As an electrical insulator, the aluminum oxide acts to prevent the various circuit elements from transmitting undesired electrical current to each other and creating a short circuit. However, it is often desirable to have an electronic substrate with a high thermal conductivity. That is to say, the substrate has the ability to dissipate the heat generated when current is passed through the circuit elements, thus providing a modicum of protection for the fairly sensitive resistors against the temperature increases, as well as preventing the burnout of circuit leads. Unfortunately, aluminum oxide has a low thermal conductivity. Additionally, it is also desirable that an electronic substrate have a low dielectric constant, i.e., the substrate should not act, in general, as a capacitor. In this regard, the dielectric constant of aluminum oxide is 9.
Dense, alumina-based ceramic materials are also known to be useful as abrasive grains. An example of alumina-based ceramic materials made by a sol-gel process is illustrated in U.S. Pat. No. 4,574,003, wherein the gel is dewatered to form a dried solid which is cut or machined to form a desired shape or crushed or broken by any suitable means to form abrasive particles or grains for industrial application.
Glass is a very important part of ceramics. The glass industry is the largest single element of the entire ceramic industry and glassy portions of many ceramic bodies are the bond that hold many ceramics together. Probably the majority of ceramics produced are a mixture of crystalline grains and a glassy phase, with the glass frequently acting as the bond. This is the basis of the vitrified-grinding wheel industry in much of the structural and whiteware branches of the ceramics industry. The cutting tool and abrasives industry is also desirous of obtaining new and better cutting and grinding tools.
Diamond, the hardest known natural material, also is an electric insulator, has a high thermal conductivity and possesses a low dielectric constant, qualities desirable and useful in connection with both abrasive applications and in electronic substrates. The use of diamond itself as either a grinding wheel or an electronic substrate is clearly cost prohibitive, whether the diamond is naturally occurring or is less expensive commercially available diamond powder or diamond chips (a by-product of processing natural diamonds) which then would need to be fused into such a grinding wheel or compressed into a substrate. Hence, it would be desirable to incorporate diamond, whether synthetic or natural, into a less expensive material, such as a ceramic, to produce a diamond-based composite that could, with appropriate modification for each application, be useful in the electronics industry or as an abrasive.
The incorporation of diamond particles into ceramics by conventional processing is extremely difficult. In such a conventional process, the raw materials, usually with clay or, with respect to ceramic substrates, aluminum oxide as the main or base material, are fired to a point where the base material undergoes a gradual change from the crystalline to the glassy state, the rate depending upon the time of heating and the temperature to which it is subjected. The fusion point of the raw materials varies from feldspar at 1300.degree. C., to kaolin (pure clay) at an order of about 1700.degree. C. to aluminum oxide at 2015.degree. C. Unfortunately, diamond decomposes to graphite (which acts as an electrical conductor and would not be useful in electronic substrates) above about 1000.degree. C. Moreover, diamond oxidizes at temperatures above about 600 C. to form carbon monoxide or carbon dioxide in the presence of oxygen. In making silica glass, for example, powdered quartz (beach sand of high quality) is melted at temperatures of between 2000.degree. and 2500. C., at which temperatures diamond is highly unstable. Additionally, conventional casting requires the use of special high temperature molds. Thus, the prior art has developed, and now uses, methods of depositing or otherwise affixing diamond or in other cases abrasive layers such as aluminum oxide only on the outer periphery of grinding wheels and other such abrasive or cutting instruments.
In general, materials that are good thermal conductors are also good electrical conductors, and hence the electronic substrate industry has yet to find a material having all three properties simultaneously, i.e., a high thermal conductivity, a low dielectric constant and the ability to act as an electric insulator.
It is a feature of the present invention to provide a material with a high thermal conductivity, a low electrical conductivity, and a low dielectric constant which is useful for electronic substrates such as hybrid circuit boards for integrated circuits.
It is another feature of the present invention to provide a very hard material having cutting and abrasive characteristics useful in grinding and cutting applications.
It is yet another feature of the present invention to provide a very hard ceramic material having high thermal conductivity characteristics, is lightweight, can be made with low temperature molds and is useful for high-temperature engines and engine parts for automobiles.